Enclosures and panels made of sheet metal are commonly used to enclose or otherwise protect electrical and electronic equipment. Such a sheet metal structure may include one or more knockouts. A knockout is a severable region of the sheet metal. A knockout comprises a region of the sheet metal having a perimeter that is partly removed or partly weakened (e.g., made frangible) with respect to the surrounding sheet metal or “parent part.” A portion of the perimeter remains intact until the knockout is removed.
To form a knockout, a punch tool, laser, or similar cutting or perforating tool can be used to cut into the surface of the sheet metal on the perimeter. To remove the knockout, a person can strike the tab with a hammer, screwdriver or other tool that delivers an impact force. As a result of the impact, an intact (e.g., frangible) portion of the perimeter fractures, and the knockout breaks free from the parent part along the perimeter. The person can then remove the knockout from the parent part. Various types of knockouts are known.
In some types of knockouts, the cut that defines the perimeter extends completely through the thickness of the sheet metal, i.e., all material along sections of the perimeter is completely removed. In other types of knockouts, the cut that defines the perimeter comprises a groove that extends only partly through the thickness of the sheet metal. That is, a cut extends from the first surface of the sheet metal to a depth between the first surface and second surfaces. Between the bottom of the cut and the second surface the perimeter is defined not by the cut but rather by a break or weakness that makes the perimeter (or sections thereof) frangible.
In some types of knockouts, the perimeter is defined by removed sections of metal or frangible sections of metal except for one or more short, narrow sections or necks at which the knockout adjoins and connects with the parent part. The neck may be too wide or otherwise too tough to readily fracture by striking the knockout with a tool. To remove such a knockout, a person must not only strike the knockout to break the knockout free along the portions of the perimeter that are removed or frangible, but the person must also then break the neck. For example, such a neck commonly can be broken by bending or twisting the knockout until the metal is so weakened or fatigued that it breaks. Once the knockout is free from the parent part at all points along the perimeter, the person can then remove the knockout from the parent part.
Removing a knockout by the impact-based method described above can undesirably deform the surrounding sheet metal of the parent part because some of the impact force is transferred to the surrounding sheet metal. This problem is especially acute in removing larger panel-shaped knockouts, as it is difficult to apply an impact force uniformly throughout the knockout.